Logstash (sometimes) doesn't process any logs after startup

I'm using logstash with:
Input plugins: `courier`, `tcp` and `pipe`
Output plugins: `elasticsearch`, `statsd`

It sometimes hangs right after it finishes starting:

```
2015-06-09_13:47:20.48056 Jun 09, 2015 1:47:20 PM org.elasticsearch.node.internal.InternalNode
2015-06-09_13:47:20.48059 INFO: [XXX-709997350-8066] version[1.5.1], pid[1], build[5e38401/2015-04-09T13:41:35Z]
2015-06-09_13:47:20.48142 Jun 09, 2015 1:47:20 PM org.elasticsearch.node.internal.InternalNode
2015-06-09_13:47:20.48144 INFO: [XXX-709997350-8066] initializing ...
2015-06-09_13:47:20.50108 Jun 09, 2015 1:47:20 PM org.elasticsearch.plugins.PluginsService
2015-06-09_13:47:20.50110 INFO: [XXX-709997350-8066] loaded [], sites []
2015-06-09_13:47:22.57233 Jun 09, 2015 1:47:22 PM org.elasticsearch.node.internal.InternalNode
2015-06-09_13:47:22.57235 INFO: [XXX-709997350-8066] initialized
2015-06-09_13:47:22.57289 Jun 09, 2015 1:47:22 PM org.elasticsearch.node.internal.InternalNode start
2015-06-09_13:47:22.57290 INFO: [XXX-709997350-8066] starting ...
2015-06-09_13:47:22.65361 Jun 09, 2015 1:47:22 PM org.elasticsearch.transport.TransportService doStart
2015-06-09_13:47:22.65363 INFO: [XXX-709997350-8066] bound_address {inet[/0:0:0:0:0:0:0:0:9300]}, publish_address {inet[/172.16.35.157:9300]}
2015-06-09_13:47:22.66330 Jun 09, 2015 1:47:22 PM org.elasticsearch.discovery.DiscoveryService doStart
2015-06-09_13:47:22.66331 INFO: [XXX-709997350-8066] elasticsearch/W5WSM3J_QDu1bK9vEmXhXw
2015-06-09_13:47:25.71965 Jun 09, 2015 1:47:25 PM org.elasticsearch.cluster.service.InternalClusterService$UpdateTask run
2015-06-09_13:47:25.71966 INFO: [XXX-709997350-8066] detected_master [White Rabbit][MAb0dadvTAi0NkKhelKs7w][XXX][inet[/172.16.34.135:9300]], added {[White Rabbit][MAb0dadvTAi0NkKhelKs7w][XXX][inet[/172.16.34.135:9300]],}, reason: zen-disco-receive(from master [[White Rabbit][MAb0dadvTAi0NkKhelKs7w][XXX][inet[/172.16.34.135:9300]]])
2015-06-09_13:47:25.72898 Jun 09, 2015 1:47:25 PM org.elasticsearch.node.internal.InternalNode start
2015-06-09_13:47:25.72899 INFO: [XXX-709997350-8066] started
2015-06-09_13:47:25.81054 Logstash startup completed
```

Jstack shows http://pastebin.com/MzQud04J

Any idea what's going on?